A common feature of most precious metal alloys used in watchmaking is their high density—higher than 10 g/cm3. In fact, the two main precious metals used in watchmaking, gold and platinum, have respective densities of around 19.3 and 21.5 g/cm3. Consequently, this makes their alloys relatively heavy.
Gradable gold alloys are usually very dense because of their high gold content, and due to the fact that they only rarely incorporate a large quantity of light elements. Moreover, if gold is alloyed with a large quantity of light elements, this usually results in fragile components.
The Ti—Au system has, however, been studied, in the equiatomic intermetallic compound TiAu zone.
In their binary or tertiary form, these alloys are known to have a shape memory effect, particularly alloys with the atomic compositions Ti50Au50 et Ti50Ni10Au40.
As regards the shape memory effect, a drop in temperature Ms has also been observed following the addition of ternary elements to Ti50Au5 alloys.
TiAuFe alloys with 5, 10, 14, 15 and 20 atomic percent of iron, and TiAuCo and TiAuCoNb alloys, have been the subject of particular studies, with no usable results for the problem.
Although the biocompatibility of these alloys is often put forward for medical applications, no composition has been proposed that is intended both to achieve 18K carat gold fineness without any surplus and acceptable mechanical properties for making external timepiece components.
EP Patent Application 2548982A1 discloses alloys containing from 50% to 99% of titanium and, more precisely, alloys containing one precious element in addition to titanium, ideally between 5 and 15% of precious metal. It is noted that a gold-titanium alloy containing 5 to 15% of gold cannot achieve 18-carat gold fineness, since the required proportion by mass of 75% of gold is not attained.
U.S. Pat. No. 4,568,398 describes Ti—Au alloys containing between 95% and 40% by mass of titanium, but wherein the rest of the composition, and particularly the low proportion of gold, precludes the alloys from achieving 18 carat gold fineness.
WO Patent Application 2008/018109A1 describes gold and titanium alloys with a fineness of between 6K and 18K, with shape memory properties, which is not desired, or superelasticity, and having elongation at break. Most of these alloys are compositions containing nickel, which is not desirable for the manufacture of external timepiece or jewellery components, which likely to come into contact with human skin. This document discloses only two 18K compositions: Ti11.23Ni13.77Au75 et Ti12.5Ni12.5Au75 by mass. These two compositions are very far from a content of 50 atomic percent of titanium, and contain nickel, making them unsuitable for use in external timepiece components.